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Fifty years ago this week, an explosion on the Apollo 13 moon mission stranded three astronauts hundreds of thousands of miles from home. You probably know that Fred Haise, Jim Lovell, and Jack Swigert made it home safely (pictured). You may not know the chemist behind the rocket engine that saved them, which began its life as an apparatus for measuring chemical reaction rates. This bonus episode of Stereo Chemistry tells the story of the engine’s design with help from two of the people who created it.

The following is the script for the podcast. We have edited the interviews within for length and clarity.

Credit: Christopher Nilsen/Purdue University

In a pintle injector rocket engine, shown in these tests with nonreacting liquids, one propellant flows through the inner tube and emerges through a small gap or holes (top), while the other propellant flows from the outer tube (middle). When both are flowing, the propellants meet (bottom) and react as they enter the rocket's combustion chamber.

Sam Lemonick: Fifty years ago this week, disaster struck NASA’s Apollo program. Apollo 13’s Fred Haise, Jim Lovell, and Jack Swigert were supposed to be the third crewed mission to the moon. Instead, they found themselves fighting for their lives 300,000 kilometers from Earth.

James A. Lovell, Jr.: OK. Houston, we’ve had a problem here.

Sam: Faulty wiring had ignited an explosion in one of the craft’s oxygen tanks, setting off a tense 3-day effort to bring the astronauts home safely. Apollo 13’s story of bravery, ingenuity, and teamwork has been told plenty of times.

But no matter how well you may think you know the tale, there’s one small but crucial part you probably haven’t heard. It’s the story of the engine that brought those astronauts home, and the chemist who invented it.

On April 13, 1970, Gerard Elverum’s pintle injector rocket engine fired for 34 seconds to put the damaged Apollo 13 spacecraft on a safe path back to Earth.

I’m Sam Lemonick and in this bonus episode of Stereo Chemistry, I’ll be sharing the story of the science and serendipity behind that mostly unsung engine. In fact, Apollo 13 is just one of the successes on the pintle injector’s résumé.

It was that same engine that landed Apollo 11’s Neil Armstrong and Buzz Aldrin safely on the moon 10 months before Apollo 13’s accident. Fifty years later, SpaceX is using pintle injector engines to ferry cargo, satellites, and soon, people, into space on reusable rockets.

The story of the engine starts in late summer 1949. Gerard, or Jerry, Elverum had just finished an undergraduate degree in physics at the University of Minnesota. Like a lot of men his age, he’d served in World War II before going to college. And he was now competing for jobs with other veterans who had done the same. So Jerry paid $25 to share a ride west to California, hoping to find better luck there.

His opportunity came in the form of a job ad looking for a chemist to work at the Jet Propulsion Laboratory. JPL at that time was making ballistic missiles for the Army. Today it’s best known for making Mars rovers and other spacecraft for NASA.

Now, Jerry had studied physics, not chemistry, but he’d taken a lot of physical chemistry courses, and he had always had a keen interest in space. He was the kind of kid who built his own telescope. So he went up to the lab to make his case.

Jerry Elverum: We got talking about cosmology and space and all kinds of other stuff.

Sam: Two and half hours later.

Jerry Elverum: So he gave me the job, which was kind of a destiny job.

Sam: Jerry’s job was to evaluate the physical properties of liquid rocket propellants. In simple terms, a liquid rocket works by reacting a fuel with an oxidizer. The gases and energy produced by that reaction propel the rocket and its cargo. If you want to learn a lot more about the wild history of rocket-propellant chemistry, check out episode 23 of the podcast. There’s a link in the description.

One thing Jerry was measuring at JPL was the rate of reactions between fuels and oxidizers. So how fast the two components react. You might remember measuring reaction rates in an introductory chemistry class. Like, maybe you mixed two chemicals in a flask and watched the solution’s color change. But the reactions of high-energy rocket propellants happen way too fast to use that method.

Instead, Jerry measured reaction rates with a device consisting of two concentric tubes, open on both ends. One chemical, say the fuel, went inside the inner tube. The other, say the oxidizer, went in the space between the inner and outer tubes. Jerry would pull the inner tube out, allowing the chemicals to mix, and his team could measure the reaction rate.

Usually.

Jerry Elverum: When I got finally working with N2O4 and pure hydrazine, the reactions were so fast that it just blew the outer tube to pieces.

Sam: So Jerry went back to the drawing board to develop a new setup that let him better control how the chemicals mixed. He attached a plate to the end of the inner tube, leaving a small gap between the end of the tube and the plate. The chemical flowing in the inner tube would hit that plate and shoot out the gap, forcing it to mix with the outer chemical in small amounts.

That kept the reaction controlled enough for the group to measure reaction rates, but it also gave Jerry an idea.

As he looked at his contraption, he realized that the basic setup might work as a rocket engine and replace the traditional design at the time, which dated back to Robert Goddard, the father of liquid rockets.

Traditional rocket engines used what people call a showerhead injector. An injector is the part that feeds the propellants into the combustion chamber. Goddard’s showerhead injector, looked like, well, a showerhead. It’s a metal plate with thousands of tiny holes that allow the fuel and oxidizer to spray into the combustion chamber and react.

The showerhead design is simple and effective, but it’s not perfect. For example, the design can allow pressure waves to form inside the combustion chamber that cause the engine to burn unevenly or even shut off. This was a big problem for rockets being developed in the late 50s and early 60s.

Jerry’s tubes offered another way of injecting fuels into a combustion chamber. His device was called a pintle injector, named after that little cover that blocks the end of the inner tube. And because of its geometry, it wasn’t susceptible to pressure waves in the chamber.

Jerry Elverum: So we built a small engine up there, and it worked like a charm.

Sam: And here’s where the serendipity happens. Right about this time, Jerry’s boss at JPL got hired away by a private company, Space Technology Laboratories. He, in turn, hired Jerry away from JPL to join him.

Jerry’s project at his new job was to build a rocket engine with a throttle, meaning you could control its thrust—kind of like an accelerator pedal in a car. And Jerry realized that he could increase or decrease the thrust his engine produced by adjusting the position of the pintle. Closer to the tube reduces the flow of propellant and reduces the engine’s thrust; farther away increases it and increases the thrust. You might actually be familiar with how a pintle injector works and not even know it: it’s the same design used in some garden hose nozzles.

Now, Jerry had one more trick up his sleeve. His pintle could control the flow of propellants at the nozzle. But he knew that he needed to go a step further and control precisely how much fuel and oxidizer reached the nozzle. If the ratios weren’t correct, the engine wouldn’t burn right. With the help of engineer Don Harvey, Jerry added adjustable valves called cavitating venturi valves that delivered those proper ratios with only a couple moving parts. Then they figured out how to link the pintle and the valves so that all the parts moved together, creating a reliable, throttleable engine.

But you might be asking, why did Jerry’s bosses ask him to design a throttleable rocket engine in the first place? A basic rocket engine has two settings: on and off. But NASA was preparing to do something no one had ever done: land astronauts on the moon.

Paul Fjeld is a writer, artist, and moon-lander expert. I asked him to explain why NASA needed a new kind of engine.

Paul Fjeld: They didn’t have a deep throttling engine, and you needed to have that to do a controlled landing. You couldn’t just kind of time it perfectly so that when you finished your massive full-out, full-throttle burn you stopped immediately a second before you touched down. That just isn’t how you land on the moon without crashing.

Sam: NASA had actually already selected a different throttling engine for the lunar lander, made by Rocketdyne. The company had made almost all of NASA’s rocket engines to that point.

Rocketdyne’s lunar module engine would use the traditional showerhead design. To throttle it, Rocketdyne proposed injecting helium gas into the propellants as they flowed to the injector. These inert atoms would basically crowd out propellant molecules and make the engine burn less powerfully.

But about a year after giving Rocketdyne the contract, NASA started to get nervous about the reliability of those engines. So they asked Space Technology Laboratories, where Jerry worked, to submit a competing engine proposal.

Gerard Elverum: We competed with Rocketdyne for about a year and a half.

Sam: And his company won. But even after it got the contract, it still took Space Technology Laboratories a couple years to sort out all the kinks. Here’s Don Harvey.

Don Harvey: So we had to calibrate each valve. Then we had to synchronize the valves with the injector because how well we do that job determines how many seconds the astronauts have while hovering over the moon.

Sam: It wasn’t easy. Don said they ran into pretty much every problem you could imagine. But the team never lost hope.

Don Harvey: We always felt positive about it. We had excellent people we were working with to keep on testing, and perfecting, and solving problems along the way, just knowing blindly, I guess, that we could solve problems.

Sam: Now back to Jerry. He remembers exactly how he felt on July 20, 1969, when Apollo 11 astronauts Neil Armstrong and Buzz Aldrin were hovering over the surface of the moon, using his engine to guide the lunar module down onto the dusty Sea of Tranquility.

[Communications from Apollo 11 moon landing start playing in background]

Gerard Elverum: It was probably the most nervous time of my life. I’m sitting there probably knowing more about the lunar lander descent engine than anybody in the United States. So while they’re going down, in my mind I’m going through all of the hundreds of test runs we did on that motor. I knew exactly what it could do, what it couldn’t do, how near to the margins we were getting.

Sam: The lander’s computer initially put Armstrong and Aldrin on a course for a rocky area. Not ideal. So Armstrong had to take control of the craft and fly a little farther to a safer spot.

Gerard Elverum: And I can remember yelling to myself when Armstrong was still trying to find a place where there wasn’t a boulder that could tip the whole thing over, and I’m saying, “Set it down, don’t abort, set it down. But you gotta set it down now, we only got a few seconds of propellant left before we have to abort.” I didn’t want them to get 8 feet off the moon and not be able to land.

Sam: And then:

Apollo 11: Houston, uh, Tranquility base here. The eagle has landed.

Gerard Elverum: That was a huge relief. I felt like the whole world was sitting on my head as that landing took place. I remember the first thought in my mind was, “This is the first time we’ve ever landed a man on another body from Earth and they can never take that event away from me for the rest of my life.’

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Sam: The engine was a huge success. Here’s writer Paul Fjeld again.

Paul Fjeld: It worked really well. And it worked on every single mission. There was never a problem.

Sam: Jerry and Don’s efforts did not go unnoticed. A reporter once asked Neil Armstrong, late in his life, what he thought was the most important engineering contribution to the Apollo program. Neil said it was that valve system that metered propellant flow into the lander’s engine.

Paul Fjeld: He thought the cavitating venturis were one of the coolest technological innovations that came out of the Apollo program. It seems like a small little thing, but it was the heart of what made the descent engine throttleable, smoothly throttleable, and efficient.

Sam: Ten months after proving itself on Apollo 11, the throttling pintle injector engine was called into service in another crucial moment, one it had not been designed for: NASA needed the engine to save the crew of the crippled Apollo 13.

Don Harvey: I was in class at USC at the time. And I got pulled out of the class by security telling me that I’ve got a phone call. I picked up and that’s when I found out there was the explosion on the Apollo 13.

Sam: That was Don.

You remember that it was faulty wiring that caused one of Apollo 13’s oxygen tanks to explode. NASA had contingency plans in case they had to abort an Apollo mission before it reached the moon. But these involved using the large engine on the service module to direct the spacecraft back to Earth’s orbit. And the service module is where that oxygen tank exploded.

So NASA decided instead to use the lander engine and its pintle injector to push Apollo 13 into a new trajectory that would slingshot it around the moon and put it on a safe course back to Earth. To give the lander engine its new instructions, NASA engineers had to quickly write code to feed to Apollo 13’s computers. Remember, no one had ever planned for something like this.

So Don went straight to Houston, where he had to make sure that the engine could survive the maneuvers. There was a fear that the material that protects the inside of the lander engine’s combustion chamber would wear away and the chamber would crack, which could cause a catastrophic failure.

Don Harvey: I remember being there. When I walked into building 45, they told me, they said, “You realize once you come in here you can’t leave until the crisis is over?” And I said, “Well, who would want to leave?”

Sam: The combustion chamber held, and the engine did its job. The astronauts fired it once to put themselves on a safe trajectory back to Earth, again after they rounded the moon to shorten their trip home and adjust their landing spot, and once more to correct their path as they got closer to Earth. More than three days after the accident, Haise, Lovell, and Swigert splashed safely down in the Pacific Ocean.

Apollo 13: Hello Houston, this is Odyssey. It’s good to see you again.

Sam: The pintle injector engine stayed in use throughout the Apollo program. After those missions ended, the design was adapted to make precision thrusters to control spacecraft and missiles.

More than 50 years after Jerry first tested his motor, the design continues to break new ground.

SpaceX’s Merlin engines use the same basic pintle injector design as the Apollo landers. The engineers who made it came to SpaceX from the same company where Jerry and Don had once worked. In 2015, a Merlin engine safely landed a SpaceX rocket that had flown to space, the first time any rocket had accomplished that task. Before SpaceX, most rockets crashed into the ocean after expending their propellants. The company has now made it routine, landing rockets dozens of times since so that they can be reused.

And all of this, from the first man on the moon to reusable rockets, is because Jerry Elverum realized he could turn an apparatus made to measure reaction rates into a rocket engine.

Stereo Chemistry is a production of C&EN, the newsmagazine of the American Chemical Society. This episode was written by Sam Lemonick and produced by Matt Davenport. It was edited by Lauren Wolf, and Sabrina Ashwell is our copy editor.

The audio clips heard in this episode come from NASA’s archives, except that last clip was from the movie “Apollo 13,” which turns 15 this year.

The music you heard during this episode was “We,” by Fjodor; “Winter,” by ANBR; and “Sidewalk Strut,” by Rex Banner.

Stereo Chemistry will be back with another episode later this month. You can subscribe on Apple Podcasts, Google Play, and Spotify.

Thanks for listening.

CORRECTION

This article was updated on April 15, 2020, to reflect that Fred Haise, not Ken Mattingly, flew aboard Apollo 13. The transcript and podcast audio were updated to include this change.